The invention relates to a reception method wherein signals supplied from a radio path are received, and each signal which arrives at a receiver is delayed by a unique delay value.
It is typical of a radio system that a signal travelling between a user and a base station does not travel straight but, depending on characteristics of the environment, propagates along a plurality of paths of different lengths from a transmitter to a receiver. Multipath propagation is mainly caused by the fact that a radio signal is reflected and scatters from obstacles on the propagation path of the signal. If signals travelling along different paths have a propagation delay of different length, it is possible for the signals to arrive at a receiver with different phases.
As a receiver solution a radio system can use a so-called RAKE receiver, which comprises one or more RAKE fingers. Each finger is an independent receiver which serves to compose and demodulate received signal components. The value of the propagation delay of a signal received by a finger can be selected such that the value corresponds to a delay caused to the radio signal by a certain propagation path. Typically, the signal components of different receiver fingers are combined, whereby a high-quality signal can be generated.
Radio systems use the CDMA method, for example, wherein a narrowband data signal is modulated by a spreading code which is more broadband than the data signal, whereby the data signal is spread over a relatively broad band. The spreading code usually comprises a long pseudorandom bit sequence. Each user of a subscriber terminal has a unique spreading code during a connection. In the CDMA system, a plurality of users can simultaneously transmit a signal over the same frequency band. The signals to be transmitted are separated from each other at the receiver on the basis of the pseudorandom spreading code. In the CDMA system, the propagation delay of the signal can be indicated as a delay of the spreading code wave form, for example.
In RAKE receivers, so-called maximal ratio combining is typically used in combining signals received from different fingers. A RAKE receiver can be applied to base stations and subscriber terminals. The subscriber terminals can be mobile telephones, for example. The RAKE receiver can be used, for example, in such a manner that it combines signals supplied from different base stations and different antenna sectors. This means that the RAKE receiver is suitable for use in soft handover and in softer handover when a transition is carried out from an antenna sector of a base station to another.
In order for a signal generated by the RAKE receiver to be of good quality, the correlators of the RAKE fingers should be optimally adjusted. A correlator of the RAKE finger is optimally adjusted when the delay of the spreading code wave form formed by the correlator corresponds as closely as possible to the propagation delay of the signal propagated on the radio path.
The propagation delay of the signal can be determined from impulse response formed from the signal received from the radio channel. The channel impulse response reveals the distribution of the total received signal power among different propagation delays. Different delay components can be calculated from the correlation between the received signal and the spreading code wave form, the correlation being determined for delays of the spreading code wave form of different lengths. The correlations can be determined for the spreading code by means of a matched filter. The matched filter enables even a large number of delay values to be determined efficiently.
A procedure of adjusting the delays of the spreading codes of the correlators of RAKE fingers to correspond to estimated values of the propagation delay of a radio wave is called RAKE finger allocation. In the prior art, RAKE fingers are allocated by setting the delays of the spreading code wave forms of the correlators to correspond to maximum power points occurring in measured impulse response.
The problem with the prior art is that finger allocation does not function efficiently in all situations. Particularly problematic are situations wherein a wide maximum power range occurs in the impulse response and the maximum power range comprises several signal components having roughly similar delays. As a result of the above-mentioned problem, part of the signal has been lost. Further problems have occurred because, for example, the tracing of the spreading code of the signal of the receiver finger has proceeded too close to the tracing of the spreading code of the neighbouring finger. It is impossible to achieve optimal finger allocation with the prior art solution since the correlator of each finger is optimized separately.
In the known solution, the code phases of the RAKE fingers are selected to correspond to the power maximums of the impulse response. Usually, the prior art solutions also use so-called code tracking to adjust the delay of the RAKE finger in an attempt to set the code phase more accurately at the maximum in the power density of the channel. Selecting the maximums and code tracking cause problems if the impulse response of the channel does not comprise maximum points clearly located separately. If code tracking moves the RAKE fingers near each other, the performance deteriorates since the fingers receive signal components propagated along the same multipaths. In such a case, the proportion of some multipaths in a maximal-ratio-combined signal becomes overemphasized and the quality of the combined signal deteriorates. The selection of the maximums may also cause part of the signal associated with the wide power maximum to be lost.
An object of the invention is thus to provide a reception method and a receiver so as to enable the above-mentioned problems to be solved. This is achieved by the reception method disclosed in the introduction, which is characterized by determining the strength of a part of a signal delayed by a certain propagation delay for several values of the propagation delay, forming sums of the strengths of the signal parts determined for different propagation values, selecting from among the received signals those signals whose delay values deviate at least by a predetermined delay difference and whose sum of the strengths of the signal parts obtains the highest possible value, indicating information by using the signal parts which correspond to the selected delay values.
This is also achieved by the reception method disclosed in the introduction, which is characterized by determining the strength of a part of a signal delayed by a certain propagation delay for several values of the propagation delay, utilising in the method a trellis diagram comprising transitions weighted by coefficients dependent on the strengths of the parts of the signal, searching for optimal routes through the trellis diagram on the basis of the sums of the weight values of the transitions, selecting a number of delay values by utilising the found optimal routes, indicating the signals received from the radio path on the basis of the parts of the signals corresponding to the selected delay values.
The invention further relates to a receiver to receive signals supplied from a radio path, the signals comprising information and each signal arriving at the receiver delayed by a unique delay value.
The receiver of the invention is characterized in that the receiver comprises a device and a distribution device to determine the strength of a part of a signal delayed by a certain propagation delay by several values of the propagation delay, the distribution device compares the delay differences of the received signals with each other and forms sums of the strengths of the signal parts determined by different values of the propagation delay, the distribution device selects from among the received signals those signals whose delay values deviate at least by a predetermined delay difference and whose sum of the strengths of the signal parts obtains the highest possible value, and the receiver indicates the received information by using the signals selected above.
Preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that optimal delays (spreading code phases) are found for the fingers of a RAKE receiver by estimating the impulse response of the channel, which is formed from the power density of the signal at feasible values of propagation delays. Furthermore, the invention is based on the idea that the optimal delay values are found on the basis of the impulse response of a measured channel by using a specific optimization algorithm. Using the optimization algorithm guarantees that the code phases selected for the RAKE fingers sufficiently deviate from each other, i.e. the fingers are at least a so-called minimum distance away from each other.
In the method of the invention, the selection of suitable code phases is based on the fact that the selected code phases meet certain optimality conditions characteristic of the invention. The conditions are set so as to enable a signal of as high quality as possible to be achieved, the signal being formed by combining signals supplied from different RAKE fingers. A trellis diagram, i.e. a network diagram, is utilised in finding the code phases which meet the optimality conditions. An optimal route determined through the trellis diagram is utilised in searching for the code phases. The weight values of the transitions in the trellis diagram are selected by utilising the impulse response measured for the radio channel.
The method and receiver of the invention provide several advantages over the known methods and receiver solutions. The problems with the selection of the maximums and code tracking are avoided by the method of the invention since the delays are selected such that the receiver catches the most part of the signal regardless of the details of the impulse response form.
The reception method can be used although the impulse response of the channel comprises only one power maximum. In a case that commonly occurs in practice, the power maximum of the impulse response comprises several multipath-propagated signal components located near each other, the existence of the signal components being indicated by the broadening of the power maximum. In the method of the invention, the power maximum is optimally distributed among several fingers if the width of the maximum is sufficient. Thus, the reception achieves sensitivity greater than that in the known solutions.
In the method of the invention, the reception of a wide impulse response peak can easily be distributed among several fingers since the minimum distance condition characteristic of the invention used in the selection of the delays of the fingers guarantees that different fingers receive signal components propagating along separate multipaths. The method of the invention enables optimal finger positions to be found. Furthermore, the optimal number of fingers of the receiver can be determined by the method.